HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of benefits to using HVDC electrical power transmission. HVDC is particularly useful for power transmission over long distances and/or interconnecting alternating current (AC) networks that operate at different frequencies.
A first station may therefore transmit electrical energy to a second station over one or more DC transmission lines, e.g. overhead lines or subsea or buried cables. The first station may generate the DC supply by conversion from a received AC input supply. The second station then typically provides conversion back from DC to AC. Each of the first and second stations may therefore typically comprise a converter for converting from AC to DC or vice versa.
Initially HVDC power transmission systems tended to be implemented for point-to-point transmission, i.e. just from the first station to the second station. Increasingly however it is being proposed to implement HVDC power transmission on a mesh-network or DC grid comprising a plurality of DC transmission paths connecting more than two voltage converters. Such DC networks are useful, for example, in applications such as electrical power generation from renewable sources such as wind farms where there may be a plurality of sources that may be geographically remote.
To date most HVDC transmission systems have been based on line commutated converters (LCCs), for example such as a six-pulse bridge converter using thyristor valves. LCCs use elements such as thyristors that can be turned on by appropriate trigger signals and remain conducting as long as they are forward biased.
Increasingly however voltage source converters (VSCs) are being proposed for use in HVDC transmission. VSCs use switching elements such as insulated-gate bipolar transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system. VSCs are thus sometime referred to as self-commutating converters.
Various designs are VSC are known. In one form of known VSC, often referred to as a six pulse bridge, each valve connecting an AC terminal to a DC terminal comprises a set of series connected switching elements, typically IGBTs, each IGBT connected with an antiparallel diode. The IGBTs of the valve are switched together to connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb (i.e. the two valves that connect the two DC terminals respectively to the same AC terminal) being switched in antiphase. By using a pulse width modulated (PWM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved.
In another known type of VSC, referred to a modular multilevel converter (MMC), each valve comprises a series of cells connected in series, each cell comprising an energy storage element, such as a capacitor, and a switch arrangement that can be controlled so as to either connect the energy storage element in series between the terminals of the cell or bypass the energy storage element. The cells or sub-modules of a valve are controlled to connect or bypass their respective energy storage element at different times so as to vary over the time the voltage difference across the valve. By using a relatively large number of sub-modules and timing the switching appropriately the valve can synthesise a stepped waveform that approximates to a sine wave and which contain low level of harmonic distortion. As will be understood by one skilled in the art there are various designs of MMC. For example an MMC may be a half-bridge MMC or a full bridge MMC. In a half-bridge MMC the energy storage element of a cell or sub-module is connected with a half-bridge switch arrangement, which allows the energy storage element to be bypassed or connected to provide a voltage of a given polarity at the terminals of the cell. In a full-bridge MMC the energy storage element of a cell or sub-module is connected with a full-bridge switch arrangement, which allows the energy storage element to be bypassed or connected to provide a voltage of either polarity at the terminals of the cell.
In normal use the VSCs of the HVDC stations are typically controlled with reference to the AC waveform of the relevant connected AC network to achieve a desired power flow. One VSC may operated in voltage control to control the voltage of the DC lines, with another VSC being controlled in a power control to control power flow.
In use the DC lines are thus charged to the relevant operating DC voltages. On initial start-up of the DC network, or in some instances on re-start after a fault, it can therefore be necessary to charge the DC lines up to the operating voltages.
Before start-up of the DC link, or following some fault conditions, the VSCs connected to the DC network may be in a blocked, non-operational, state. Typically one VSC is used as an energising converter and is de-blocked in voltage control mode and used to charge the DC line(s), with the other converter(s) remaining in the blocked state. The energising converter thus charges a DC line at its proximal end where the DC line may effectively be open-circuited at its distal end. This can result in voltage oscillations in the DC line that can result in a voltage magnitude at the distal end that is greater than 1 p.u. and which may significantly exceed the rated voltage of the DC link, which is undesirable.